It is known that polyolefins have excellent moldability, rigidity, hygienic properties and impact resistance and that an addition of a clarifying agent thereto reduces the opacity of the resulting molded article and thus improves the transparency; therefore, polyolefins are used in a variety of applications such as food containers, beverage containers, protective sheets and wrapping materials.
Generally speaking, consumers tend to prefer highly transparent products in many applications. In the case of storage containers where consumers desire to see the container contents, the more transparent the container is, the greater becomes the appealing power of the product and the higher can be the asking price thereof in the storage container market.
However, even if individual molded article is colorless and transparent, for example, when water is filled therein, the background of the molded article is distorted and there is thus a problem of insufficient transparency.
Further, in order to impart a molded article with functions such as heat resistance and weather resistance, additives such as an antioxidant, an acid removing agent, a slip agent, a brightening agent, an ultraviolet absorber and a hindered amine compound are blended; however, since addition of these additives may cause the resulting molded article to be colored, an improvement is demanded in this respect.
For example, in Patent Document 1, for the purpose of improving the transparency of a molded article, a composition in which 50 to 10,000 parts of a water-insoluble colorant is added to 1,000,000 parts of a clarifying agent is disclosed. In addition, Patent Document 2 discloses a polymer composition in which a polymeric nucleating agent, a propylene polymer and a color pigment are blended.
Meanwhile, in polymers such as polyolefins, it is known that their molecules are each in the form of a nano-sized string. In an amorphous state, these polymer molecules have no bending directionality and thus serve as a homogeneous medium for light as well; however, when the polymer is molded by injection molding or the like, the polymer molecules assume an oriented state. In such an oriented state, a birefringence phenomenon appears due to the difference between the refractive index (n//) for light linearly polarized in the orientation direction and the refractive index (n⊥) for light linearly polarized in the direction perpendicular to the orientation direction. Particularly, when a plastic bottle such as a beverage container is filled with water, this birefringence phenomenon is prominent and the transparency of the plastic bottle is impaired in some cases.
Further, in the case of plastic bottles where a material through which the product contents are visible is demanded, even if the plastic bottles are themselves transparent, there is a problem that they do not appear transparent when put on store-front display.
As a method improving the transparency of an olefin molded article, a variety of methods have been examined and, for example, Patent Document 1 and 2 disclose polypropylene-based resin films for surface protection which comprise a nucleating agent in an amount of 100 ppm or less and have a birefringence (Δn) of 0.4×10−3 to 2.5×10−3.